Ni-BASED COMPOUND SUPERALLOY HAVING EXCELLENT OXIDATION RESISTANCE, METHOD FOR MANUFACTURING THE SAME, AND HEAT-RESISTANT STRUCTURAL MATERIAL

ABSTRACT

The present invention is characterized in including Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, and having a multi-phase microstructure including a primary L1 2  phase and an (L1 2  phase+D0 22  phase and/or D0 24  and/or D0 a  phase) eutectoid microstructure.

TECHNICAL FIELD

The present invention relates to a Ni-based compound superalloy havingexcellent oxidation resistance, which has a multi-phase microstructureincluding a primary L1₂ phase and an eutectoid microstructure (L1₂phase+D0_(x) phase (including D0₂₂ phase, D0₂₄ phase, or D0_(a) phase)).The present invention further relates to a method for manufacturing theaforementioned Ni-based compound superalloy.

This application claims priority from Japanese Patent Application No.2006-261569, filed on Sep. 26, 2006, the content of which isincorporated herein by reference.

BACKGROUND ART

Nowadays, most high-temperature structural materials for turbinecomponents of jet engines or gas turbines are Ni-based superalloys.Because at least approximately 35 vol % or more of the constituentphases of Ni-based superalloy are metal phases (γ), there arelimitations in melting point and high-temperature creep strength ofNi-based superalloys. As candidates for high-temperature structuralmaterials that surpass the Ni-based superalloys, high-temperaturestructural materials including intermetallic compounds in which theyield stress shows positive temperature dependence can be raised.However, single-phase materials have drawbacks of poor ductility at roomtemperature and low creep strength at high temperature. As tomulti-phase materials compared with single-phase materials, because anyof Ni₃X type intermetallic compounds has a GCP (Geometrically ClosePacked) crystal structure, there is a possibility that some of theseintermetallic compounds may be combined with high coherency. Since thereare a number of Ni₃X type intermetallic compounds that have superiorproperties, by forming Ni₃X type intermetallic compounds in the form ofa multi-phase material, a new type of multi-phase intermetalliccompounds, that is, multi intermetallics, having further excellentproperties and a high freedom for microstructural control are expectedto be produced.

It was previously reported that an attempt has been made to develop amulti-phase compound using a Ni₃Al(L1₂)-Ni₃Ti(D0₂₄)-Ni₃Nb(D0_(a))system, and an alloy having superior properties could be developed (seeNon-Patent Document 1). (Non-Patent Document 1)

K. Tomihisa, Y. Kaneno, T. Takasugi, Intermetallics, 10 (2002) 247

DISCLOSURE OF THE INVENTION Problems To Be Resolved by the Invention

The aforementioned Ni-based superalloys are employed as structuralmaterials for engines and the like where high-temperature heatresistance is required. In engines where this type of material isapplied, the engine efficiency is influenced by the operatingtemperature and the engine weight. The density of the aforementionedNi-based superalloy is 8.0 to 9.0 g/cm³, which is relatively heavy.Accordingly, there has been progress in the development of a Ni-basedcompound superalloy that has a slightly lighter specific gravity thanthat of the aforementioned Ni-based superalloy.

With this in mind, the present inventors carried out research anddevelopment with the goal of developing a superalloy having even moresuperior properties than these conventional Ni-based superalloys. As oneaspect of these efforts, the present inventors carried out research anddevelopment of a Ni-based compound superalloy which includes Al in theamount of 5 to 13 at %, V in the amount of 9.5 to 17.5 at %, Ti in theamount of 0 to 3.5 at %, B in the amount of 1000 ppm (weight) or less,and Ni as the remainder, and has a dual multi-phase microstructureincluding a primary L1₂ phase and an (L1₂ phase+D0₂₂ phase) eutectoidmicrostructure.

The density of this Ni-based compound superalloy is in the range of 7.5to 8.5 g/cm³, and is lighter in weight than the previously mentionedNi-based superalloy. This Ni-based compound superalloy also has roughlythe same high-temperature strength at temperatures up to around 1 000°C. as the aforemented Ni-based superalloy.

However, the aforementioned Ni-based compound superalloy is problematicin that its oxidation resistance is inferior.

In order to solve the aforementioned problems, the present inventionaims to provide a Ni-based compound superalloy that is lighter in weightthan the Ni-based superalloy, has roughly the same high-temperaturestrength at temperatures up to around 1000° C. as the Ni-basedsuperalloy, and, moreover, has superior resistance to oxidation.

Means to Resolve the Problems

The present invention employs the following design to achieve the aboveaims.

-   (1) One aspect of the Ni-based compound superalloy having excellent    oxidation resistance according to the present invention includes:    Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at    % or less; and Ti: 0 at % or more to 3.5 at % or less, with the    remainder being Ni and unavoidable impurities, and has a multi-phase    microstructure including a primary L1₂ phase and an (L1₂ phase+D0₂₂    phase and/or D0₂₄ and/or D0_(a) phase) eutectoid microstructure.-   (2) The Ni-based compound superalloy having excellent oxidation    resistance according to the present invention may further include    Nb: 3 at % or more to 9.5 at % or less, and the amount of V may be    not less than the amount of Nb.-   (3) Another aspect of the Ni-based compound superalloy having    excellent oxidation resistance according to the present invention,    has a multi-phase microstructure including a primary L1₂ phase and    an (L1₂ phase +D0₂₂ phase and/or D0₂₄ and/or D0_(a) phase) eutectoid    microstructure, which has a composition within the limits which link    point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %),    point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at    %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75    at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni:    75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %,    Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram    shown in FIG. 2.-   (4) The Ni-based compound superalloy having excellent oxidation    resistance according to the present invention may further include at    least one or more of Co: 15 at % or less and Cr: 5 at % or less.-   (5) The Ni-based compound superalloy having excellent oxidation    resistance in any one of (1), (2) and (4) according to the present    invention may further include B: 1000 ppm (weight) or less.-   (6) The Ni-based compound superalloy having excellent oxidation    resistance according to the present invention may have a dual    multi-phase microstructure including the primary L1₂ phase and the    (L1₂ phase+D0₂₂ phase and/or D0₂₄ and/or D0_(a) phase) eutectoid    microstructure.-   (7) The heat-resistant structural material having excellent    oxidation resistance according to the present invention includes the    Ni-based compound superalloy according to any one of (1) to (6).-   (8) One aspect of the method for manufacturing a Ni-based compound    superalloy having excellent oxidation resistance according to the    present invention, includes: subjecting an alloy material containing    Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at    % or less; and Ti: 0 at % or more to 3.5 at % or less, with the    remainder being Ni and unavoidable impurities, to a first heat    treatment at a temperature at which a primary L1₂ phase and an Al    phase coexist; and thereafter cooling the alloy material to a    temperature at which the primary L1₂ phase and a D0₂₂ phase and/or a    D0₂₄ phase and/or a D0_(a) phase coexist, or further subjecting the    alloy material to a second heat treatment at this temperature,    thereby converting the Al phase to an (L1₂ phase+D0₂₂ phase and/or    D0₂₄ phase and/or D0_(a) phase) eutectoid microstructure to form a    multi-phase microstructure.-   (9) In the method for manufacturing a Ni-based compound superalloy    having excellent oxidation resistance according to the present    invention, the alloy material may further include Nb: 3 at % or more    to 9.5 at % or less, and the amount of V may be not less than the    amount of Nb.-   (10) Another aspect of the method for manufacturing a Ni-based    compound superalloy having excellent oxidation resistance according    to the present invention, includes: subjecting an alloy material    having a composition within the limits which link point A (Al: 14.0    at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B (Al: 12.5    at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C (Al: 8.0    at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D (Al:    2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and point E    (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in the    Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2, to a    first heat treatment at a temperature at which a primary L1₂ phase    and an Al phase coexist; and thereafter cooling the alloy material    to a temperature at which the primary L1₂ phase and a D0₂₂ phase    and/or a D0₂₄ phase and/or a D0_(a) phase coexist, or further    subjecting the alloy material to a second heat treatment at this    temperature, thereby converting the Al phase to an (L1₂ phase+D0₂₂    phase and/or D0₂₄ phase and/or D0_(a) phase) eutectoid    microstructure to form a multi-phase microstructure.-   (11) In the method for manufacturing a Ni-based compound superalloy    having excellent oxidation resistance according to the present    invention, the alloy material may further include at least one or    more of Co: 15 at % or less, and Cr: 5 at % or less.-   (12) In the method for manufacturing a Ni-based compound superalloy    having excellent oxidation resistance according to the present    invention, the alloy material may further include B: 1000 ppm or    less.-   (13) In the method for manufacturing a Ni-based compound superalloy    having excellent oxidation resistance according to the present    invention, the first heat treatment may be carried out at a    temperature at which the alloy material is in a first state shown in    FIG. 1.-   (14) In the method for manufacturing a Ni-based compound superalloy    having excellent oxidation resistance according to the present    invention, the second heat treatment may be carried out at 1173 K to    1273 K.

EFFECTS OF THE INVENTION

The present invention provides a Ni-based compound superalloy whichincludes: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with theremainder being Ni and unavoidable impurities, and has a multi-phasemicrostructure including a primary L1₂ phase and an (L1₂ phase+D0₂₂phase and/or D0₂₄ and/or D0_(a) phase) eutectoid microstructure. As aresult, the Ni-based compound superalloy according to the presentinvention has a specific gravity that is slightly less than that of theconventional Ni-based superalloy, superior high-temperature strength attemperatures up to around 1000° C. that is on par with the Ni-basedsuperalloy, and superior resistance to oxidation.

The manufacturing method according to the present invention enables themanufacturing of a Ni-based compound superalloy having a multi-phasemicrostructure including a primary L1₂ phase and an (L1₂ phase+D0₂₂phase and/or D0₂₄ and/or D0_(a) phase) eutectoid microstructure, thisNi-based compound superalloy has a specific gravity that is slightlyless than that of the conventional Ni-based superalloy, a superiorhigh-temperature strength at temperatures up to around 1273 K (1000° C.)that is on par with a Ni-based superalloy, and superior resistance tooxidation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal phase diagram related to Al contents andtemperatures in the case in which the Ti content is 2.5 at % for onespecific example of an alloy having the composition system which servesas the base for the Ni-based compound superalloy according to thepresent invention.

FIG. 2 is a Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram at 1273 Kwhich is formed from various specific examples of the Ni-based compoundsuperalloys according to the present invention and alloys having thecomposition systems which serve as the base therefor.

FIG. 3 is a graph of the results of a compression test showing therelationship between temperature and yield stress for various testmaterials obtained from specific examples of the Ni-based compoundsuperalloy according to the present invention.

FIG. 4 is a graph of the results of oxidation tests showing therelationship between the amount of weight increase and exposure time forvarious test materials obtained from specific examples of the Ni-basedcompound superalloy according to the present invention.

FIG. 5A are photos of microstructures of Test Materials No. 21. 22 and23 produced in the Examples.

FIG. 5B is a photo (5000-fold magnification) of a metallographicstructure of Test Material No. 21 produced in the Examples.

FIG. 6 is a photo (1000-fold magnification) of a metallographicstructure of Test Material No. 28 produced in the Examples.

FIG. 7 is a photo of a metallographic structure of the same testmaterial photographed after changing the field of view.

FIG. 8 is a photo of a metallographic structure in which a portion ofthe multi-phase microstructure of the same material is photographed at2500-fold magnification.

FIG. 9 is a graph showing the results of the tests of oxidationresistance for the various test materials.

FIG. 10 is a graph of the results of oxidation tests showing therelationship between increase in mass and exposure time for TestMaterials No. 41 to 48 obtained from specific examples of the Ni-basedcompound superalloy according to the present invention.

FIG. 11 is a graph of the results of oxidation tests showing therelationship between increase in mass and exposure time for TestMaterials No. 51 to 58 obtained from specific examples of the Ni-basedcompound superalloy according to the present invention.

FIG. 12 is a graph of the results of oxidation tests showing therelationship between increase in mass and exposure time for TestMaterials No. 63 to 67 obtained from specific examples of the Ni-basedcompound superalloy according to the present invention.

FIG. 13 is a graph showing the results of tensile tests for TestMaterials No. 28, 41, and 65.

FIG. 14 is a photo of a microstructure of Test Material No. 41 which isphotographed at 1000-fold magnification.

FIG. 15 is a photo of a microstructure of Test Material No. 41 which isphotographed at 5000-fold magnification.

FIG. 16 is a photo of a microstructure of Test Material No. 47 which isphotographed at 5000-fold magnification.

FIG. 17 is a photo of a microstructure of Test Material No. 48 which isphotographed at 5000-fold magnification.

FIG. 18 is a photo of a microstructure of Test Material No. 52 which isphotographed at 2500-fold magnification.

FIG. 19 is a photo of a microstructure of Test Material No. 57 which isphotographed at 2500-fold magnification.

FIG. 20 is a photo of a microstructure of Test Material No. 65 which isphotographed at 50-fold magnification.

FIG. 21 is a photo of a microstructure of Test Material No. 65 which isphotographed at 1000-fold magnification.

FIG. 22 is a photo of a microstructure of Test Material No. 65 which isphotographed at 5000-fold magnification.

FIG. 23 is a stress-strain diagram showing the results of tensile testson the various tests materials obtained by adding various amounts of Bto Test Material No. 65.

FIG. 24 is a photo of a microstructure of the test material obtained bysubjecting a test material in which 25 ppm of B has been added to TestMaterial No. 65 to a homogenizing treatment at 1300° C. for 3 hours.

FIG. 25 is a photo of a microstructure of the test material obtained bysubjecting a test material in which 25 ppm of B has been added to TestMaterial No. 65 to a homogenizing treatment at 1330° C. for 3 hours.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be explained using theaccompanying figures. However, the present invention is not limited tothe various embodiments explained below.

The Ni-based compound superalloy according to the present inventionincludes: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with theremainder being Ni and unavoidable impurities, wherein the amount of Vis not less than the amount of Nb, and the Ni-based compound superalloyhas a multi-phase microstructure including a primary L1₂ phase and an(L1₂ phase+D0₂₂ phase and/or D0₂₄ and/or D0_(a) phase) eutectoidmicrostructure.

The Ni-based compound superalloy according to the present invention mayinclude Co: 15 at % or less in addition to the above composition, andmay include Cr: 5 at % or less in addition to the above composition, andalso may include B: 1000 ppm (weight) or less in addition to the abovecomposition. Further, in addition to the above composition, it ispreferable that the Ni-based compound superalloy according to thepresent invention has a multi-phase microstructure including a primaryL1₂ phase and an (L1₂ phase+D0₂₂ phase and/or D0₂₄ and/or D0_(a) phase)eutectoid microstructure, and it is most preferable that the Ni-basedcompound superalloy according to the present invention has a dualmulti-phase microstructure composed of a primary L1₂ phase and an (L1₂phase+D0₂₂ phase and/or D0₂₄ and/or D0_(a) phase) eutectoidmicrostructure.

The thus-described Ni-based compound superalloy can be manufactured bythe method which includes: melting an alloy material having acomposition that includes: Al: more than 5 at % to 13 at % or less; V: 3at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % orless, with the remainder being Ni and unavoidable impurities, whereinthe amount of V is not less than the amount of Nb; carrying out a solidsolution treatment (homogenizing treatment); then carrying out a firstheat treatment at a temperature at which the primary L1₂ phase and an Alphase coexist; and then cooling the alloy material to a temperature atwhich the primary L1₂ phase and a D0₂₂ phase and/or a D0₂₄ phase and/ora D0_(a) phase coexist, or further subjecting the alloy material to asecond heat treatment at this temperature, thereby converting the Alphase to an (L1₂ phase+D0₂₂ phase and/or D0_(a) phase) eutectoidmicrostructure to form a multi-phase microstructure.

FIG. 1 is a longitudinal phase diagram of the alloy related to thecomposition system according to the present invention. In FIG. 1, theamount of Al (at %) is shown on the horizontal axis, and the absolutetemperature (K) is shown on the vertical axis. In the phase diagramshown in FIG. 1, the amount of Ti is 2.5 at %, and the amount of V is(22.5−amount of Al) at %. FIG. 2 is a Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternaryphase diagram at 1273 K made up from the results of various specificexamples related to the composition system according to the presentinvention.

The phrase “carrying out a solid solution heat treatment (homogenizingheat treatment)” as used in the present embodiments means heating to andmaintaining at the temperatures in the range indicated by Al in FIG. 1.In the case of Al: 5 to 10 at %, for example, this would be thetemperatures between the symbols “570 ” and the symbols “Δ” in theregion indicated by Al.

In the present embodiments, the alloy material may be first subjected toa solid solution heat treatment (homogenization heat treatment). Thehomogenization heat treatment is typically carried out at a highertemperature than that of a first heat treatment which is performedsubsequently. The homogenization heat treatment is preferably carriedout at a temperature in the range of 1523 to 1623 K. Here, the firstheat treatment and the homogenization heat treatment may be carried outtogether.

In the present embodiments, the alloy is subjected to the homogenizationheat treatment, and then is subjected to the first heat treatment. Thefirst heat treatment is carried out at a temperature at which both ofthe primary L1₂ phase and the Al phase coexist. The temperature at whichthe primary L1₂ phase and the Al phase coexist is specifically thetemperature at which the alloy is in the Al+L1₂ state shown in FIG. 1,that is, the temperature between the symbols “Δ” and the symbols “∘” inthe case of Al: 5 to 10 at % shown in FIG. 1.

In the present embodiments, the phrase “the first heat treatment iscarried out at a temperature at which both of the primary L1₂ phase andthe Al phase coexist” means carrying out a heat treatment in the regiondescribed as Al+L1₂ in FIG. 1. The L1₂ phase is a Ni₃Al typeintermetallic compound phase, and the Al phase is a fcc type Ni solidsolution phase.

Due to these states, from the results of the Examples below, it isassumed that the Al phases exist in between the cuboidal or rectangularprimary L1₂ phases in the microstructure. This type of microstructureincluding the primary L1₂ phases and the intervening phases can bereferred to as “upper multi-phase microstructure”.

The time for carrying out this first heat treatment is not particularlyrestricted. However, it is desirable to carry out the first heattreatment over a time period sufficient for the entire alloy to become amicrostructure including the primary L1₂ phase and the Al phase. Thetime period for carrying out the first heat treatment is, for example, 5to 20 hours.

The phrase “carrying out a second heat treatment in a region indicatedby L1₂+D0₂₂ on the alloy material which is already subjected to thefirst heat treatment” means carrying out a heat treatment, for example,at a temperature not more than temperatures indicated by the symbols “”in FIG. 1 in the case of Al: 5 to 10 at %. The temperatures at the “”symbols in FIG. 1 are 1281 K; however, these temperatures vary dependingon the composition of the alloy. The primary L1₂ phase is almostentirely unaffected by the second heat treatment. However, the Al phasedecomposes into a L1₂ phase and a D0₂₂ phase and/or a D0₂₄ phase and/ora D0_(a) phase. A multi-phase microstructure mainly including the L1₂phase and the D0₂₂ phase and/or the D0₂₄ phase and/or the D0_(a) phasewhich is provided by the decomposition of the Al phase is hereinafterreferred to as “lower multi-phase microstructure”.

In the case in which the second heat treatment is carried out after thefirst heat treatment, cooling may be accomplished by natural cooling orforcible cooling such as water-quenching. The natural cooling may becarried out, for example, by taking out the alloy material from aheat-treatment furnace after the first heat treatment and then allowingthe resulting alloy material to be put at room temperature, or byturning off a heater of the heat-treatment furnace after the first heattreatment and then allowing the resulting alloy material to be put inthe heat-treatment furnace.

A temperature for the second heat treatment is, for example, about 1173K to about 1281 K. A period for the second heat treatment is, forexample, about 5 to 20 hours, for example. The Al phase may bedecomposed into the L1₂ phase and the D0₂₂ phase by the cooling such asthe simply water-quenching and the like without the second heattreatment. However, the decomposition can be more reliably achieved bythe heat treatment at the relatively high temperature. After the secondheat treatment, the resulting alloy material may be cooled to the roomtemperature by natural cooling or forcible cooling. Note that the word“to” expressing a range as used in the present specification includesthe boundary values of the range unless otherwise described.

The reasons for limiting the various components of the Ni-based compoundsuperalloy according to the present invention will now be explainedbelow.

As is clear from the longitudinal phase diagram in FIG. 1, the phasediagram in FIG. 2, and the specific examples that follow below, thereasons for defining Al: more than 5 at % to 13 at % or less, and V: 3at % or more to 9.5 at % or less, are that, within these ranges, thefirst heat treatment can be carried out at a temperature at which theprimary phase L1₂ and the Al phase coexist, and it is possible to coolto a temperature at which the L1₂ phase and the D0₂₂ phase and/or theD0₂₄ phase and/or the D0_(a) phase coexist, or further to carry out thesecond heat treatment at this temperature, so that the multi-phasemicrostructure can be formed.

The amount of Nb may be in the range of 3 at % or more to 9.5 at % orless, and may be equal to, or less than the amount of V. To restate, theamount of V must be equal to or greater than the amount of Nb. This isbecause in the Ni-based compound superalloy of the present embodiments,a portion of V is substituted by Nb in order to improve the property ofresistance to oxidation. Resistance to oxidation improves more as theamount of the V portion substituted with Nb increases. Note that theNi-based compound superalloy of the present embodiments includes asmaller amount of V, includes Nb, and includes a larger amount of Al, ascompared to the Ni-based compound superalloy which was researched by thepresent inventors and includes Al: 5 to 13 at %, V: 9.5 to 17.5 at %,Ti: 0 to 3.5 at %, and B: 1000 ppm (weight) or less, with the remainderbeing Ni, and has a dual multi-phase microstructure including a primaryL1₂ phase and an (L1₂+D0₂₂ and/or D0₂₄ phase and/or D0_(a) phase)eutectoid microstructure. These are the different features.

Co and Cr are elements that contribute to improving resistance tooxidation. Co is preferably added in the range of 0 at % or more to 15at % or less, and Cr is preferably added in the range of 0 at % or moreto 5 at % or less.

Co is an element which has complete solid solubility in Ni, so that Cois soluble in intermetallic compounds, Ni₃Al, Ni₃V, (Ni₃Ti), and thelike. In order to maintain the characteristics of a Ni-based alloy, theadded amount is set to be up to 15 at %.

Cr is effective of improving resistance to oxidation. However, becausethe solid solubility of Cr in Ni₃Al is low, there is a concern thatunnecessary precipitates will be generated if Cr is added in a quantityof more than 5 at %. Accordingly, it is preferable to set the upperlimit for addition of Cr to be 5 at %.

The bonding strength of V with oxygen is high, so that the surface ofthe alloy material readily oxidizes. Accordingly, by decreasing theamount of V, it is possible to improve resistance to oxidation. At thesame time, V can be substituted with Nb which has the same valencenumber. Further, by increasing the amount of Al, it is possible togenerate a fine oxidized film of alumina on the surface. By decreasingthe amount of V, resistance to oxidation can be improved. However, ifthe amount of Nb exceeds the amount of V, it becomes difficult to obtaina multi-phase microstructure. Accordingly, it is necessary to increasethe amount of V to be greater than the amount of Nb.

The amount of Ti is in the range of 0 at % or more to 3.5 at % or less,preferably in the range of 0.5 to 3.5 at % or less, more preferably inthe range of 1 to 3.5 at %, and most preferably in the range of 2 to 3at %. It is preferable that the Ni-based compound superalloy accordingto the present invention includes Ti; however, it is also acceptable notto include Ti.

The amount of Ni is preferably in the range of 73 to 77 at %, and morepreferably in the range of 74 to 76 at %. This is because, in thisrange, the amount of Ni: the total amount of (Al, Ti, and V) approachesnearly 3:1, and therefore, a solid solution phase of Ni, Al, Ti, or V isessentially non-existent.

The amount of B is in the range of 0 ppm (weight) or more to 1000 ppm(weight) or less, preferably in the range of 1 to 1000 ppm (weight),more preferably in the range of 1 to 500 ppm (weight), and even morepreferably in the range of 5 to 100 ppm (weight). It is preferable thatthe Ni-based compound superalloy according to the present inventionincludes B; however it is also acceptable that B is not included.

In addition to the various elements of the above composition, it is alsoacceptable to include Mo in the amount of 1 to 2 at %. Mo is an elementthat has the effect of improving high-temperature strength, and hascomplete solid solubility in V. The amount of Mo preferably satisfiesV>Mo+Nb. Further, the method for strengthening the crystal grainboundary may be considered as an approach for improving ductility. Forthis purpose, trace quantities of elements such as C, Zr, and Hf may beadded up to a maximum of 0.2 at %. It is also acceptable to include anyone of elements C, Zr and Hf in a trace amount of 0.2 at % or less.

The Ni-based compound superalloy according to the present invention hasa multi-phase microstructure which includes an upper multi-phasemicrostructure and a lower multi-phase microstructure as describedabove, and it is most preferable that this Ni-based compound superalloyincludes a dual multi-phase microstructure including these multi-phasemicrostructures.

It will be demonstrated experimentally in the Examples which will followbelow that the Ni-based compound superalloy according to the presentinvention has superior mechanical properties at high temperatures andsuperior resistance to oxidation. It is thought that the reason forthese superior properties is because the Ni-based compound superalloyaccording to the present invention has the multi-phase microstructurethat includes the upper multi-phase microstructure and the lowermulti-phase microstructure and, the having of the aforementioned dualmulti-phase microstructure of the upper multi-phase microstructure andthe lower multi-phase microstructure, which is the more preferablefeature, is thought to be a contributing factor to attain more superiorcharacteristics.

Note that it is desirable that the multi-phase microstructure or thedual multi-phase microstructure forms the entire Ni-based compoundsuperalloy according to the present invention; however, it is notnecessary that the entire Ni-based compound superalloy has thismicrostructure. Rather, it is acceptable that at least a portion, ormore preferably 50% or more, of the entire microstructure be composed ofthe multi-phase microstructure.

The crystal structures of the intermetallic compounds employed in theNi-based compound superalloy according to the present invention aresimple as compared to the other three constituent phases (D0₂₂ phase,D0₂₄ phase, and D0_(a) phase). As a result, it is thought that theNi-based compound superalloy according to the present invention includesa primary phase L1₂ in which dislocations are comparatively activated,and a certain degree of ductility occurs over an entire range oftemperatures including a room temperature. Accordingly, this facilitateshandling of the Ni-based compound superalloy.

The Ni-based compound superalloy according to the present invention hassuperior mechanical properties at high temperatures. Accordingly, it canbe used as a heat resistant structural material. Further, among thecomponent elements, a portion of V is substituted by Nb; thereby,improving the resistance to oxidation. Further, by adding Co and Cr insuitable quantities, resistance to oxidation is also increased.

In addition, in the case in which a composition is provided in which aportion of V is substituted by Nb, it is disadventageous to some extentfrom the perspective of reducing weight; however, there is a weightreduction on the order of about 0.5 g/cm³ as compared to the typicalNi-based superalloy.

The above-described Ni-based compound superalloy can be effectivelyutilized in a temperature range that is slightly lower than 1523 K(1250° C.), for example, at high temperatures up to 1273 K to 1373 K(1000 to 1100° C.), and is suitable for low-pressure turbine blades of aturbo charger or an engine. In the case in which the high-temperaturestrength is high in this temperature range, the effect of achieving thesame resistance to pressure at a lower weight can be realized. Thus,this is beneficial from the perspective of engine efficiency and fuelcosts.

Examples of the alloy material employed to manufacture the Ni-basedcompound superalloy according to the present invention include a castingmaterial, a forging material, a single crystal material, and the like.The casting material can be formed by melting (arc melting, highfrequency melting, and the like) a pre-weighed raw metal, then pouringit into a casting mould, and permitting it to solidify.

The casting material is a polycrystal typically having crystal grains onthe order of several hundred microns to several millimeters, and has adisadvantage of readily fracturing at boundaries between the crystalgrains (crystal grain boundaries), and a disadvantage of having castingdefects such as shrinkage cavities and the like. The forging materialimproves on these disadvantages. The forging material is formed bysubjecting a casting material to a hot forging and a recrystallizationannealing. These hot forging and recrystallization annealing aretypically carried out at temperatures which are higher than thetemperature of the first heat treatment.

The temperatures at which the hot forging and the recrystallizationannealing are carried out may be the same or different. It is preferableto carry out the hot forging at around 1523 to 1623 K, and therecrystallization annealing at around 1423 to 1573 K. Prior to the firstheat treatment, the alloy material may be subjected to a homogenizationheat treatment. The homogenization heat treatment is typically carriedout at a temperature which is higher than that of the first heattreatment. The homogenization heat treatment is preferably carried outin the range of around 1523 to 1623 K. The first heat treatment may becarried out together with the homogenization heat treatment. In the caseof the forging material, the hot forging and the recrystallizationannealing may be carried out together with the homogenization heattreatment. The time period for carrying out the homogenization heattreatment is not restricted; however, for example, it is on the order of24 to 96 hours. In the case in which the alloy material is a polycrystalmaterial (casting material, forging material, or the like), it ispreferable to include B in the alloy material. The reason for this isbecause the crystal grain boundaries are strengthened as a result.

If a compression testing and a tensile testing are carried out to aNi-based compound superalloy having a multi-phase microstructure whichis formed by heat-treating a casting material, a forging material, or asingle crystal material, it can be confirmed that the Ni-based compoundsuperalloy has superior mechanical properties on any of these testing.

Examples

Various specific examples of the Ni-based compound superalloy accordingto the present invention will now be explained.

In the following examples, Ni-based compound superalloys havingmulti-phase microstructures were manufactured by carrying out heattreatments, and the mechanical properties thereof were investigated.

In the following examples, the heat treatment at 1373 K corresponds tothe first heat treatment (primary precipitation heat treatment) at atemperature at which the primary L1₂ phase and the Al phase coexist(first state), and the water-quenching carried out after performing theheat treatment at 1373 K corresponds to cooling to a temperature atwhich the L1₂ phase and the D0₂₂ phase coexist. The heat treatment at1173 K or 1273 K carried out after performing the heat treatment at 1373K corresponds to the second heat treatment (secondary precipitation heattreatment) at a temperature at which the L1₂ phase and the D0₂₂ phasecoexist.

Method for Producing Casting Material

Prior to producing test materials employing the composition systemaccording to the present invention, Ni, Al, Ti, and V raw metals (eachhaving 99.9 wt % purity) in the proportions indicated in Nos. 1 to 20 inTable 1 were melted in an arc melting furnace for obtaining castingmaterials for prescribing the composition limits of alloys resemblingthe present invention. With regard to the atmosphere inside the arcmelting furnace, the melting chamber was evacuated and then theatmosphere was replaced with an inert gas (argon gas). A non-consumabletungsten electrode was employed for the electrode, and a water-cooledcopper hearth was employed for the casting mold. In the case of addingother elements in addition to the above, it is acceptable to use rawmetals in which elements such as Co, Cr, Mo, B, C, Hf, and the like areadded in accordance with the required alloy composition, or to addingots of these elements separately during melting.

In the following explanation, the aforementioned casting materials willbe referred to as “samples”.

For actually manufacturing the Ni-based compound superalloy according tothe present invention, Ni, Al, Ti, and V raw metals were employed toobtain samples so as to produce Test Materials Nos. 1 to 20 having thevarious compositions shown in Table 1, in order to obtain a phasediagram of the basic composition system of the Ni-based compoundsuperalloy according to the present invention.

From the longitudinal phase diagram in FIG. 1, it may be understood thata sample having a composition in which the amount of Al is in a rangefrom more than 5 at % to 13 at % or less becomes to have a Ni-basedsuperalloy microstructure which is Al+L1₂ phase at 1373 K, and thatcooling to a temperature not more than the eutectoid temperature (1281K) results in the occurrence of a eutectoid reaction which isAl→L1₂+D0₂₂, D0₂₄, D0_(a), and formation of a dual multi-phasemicrostructure including a primary L1₂ phase and an (Li₂+D0₂₂, D0₂₄,D0_(a)) eutectoid microstructure.

TABLE 1 Test Sample Composition L1₂(Ni ₃ Al) D0₂₄(Ni ₃ Ti) Material (at%) Microstructure (at %) (at %) No. Ni Al Ti V at 1273 K Ni Al Ti V NiAl 1 75 2.5 17.5 5 D0₂₄ — — — — 72.9 2.3 2 75 2.5 12.5 10 rho + D0₂₂ — —— — — — 3 75 2.5 7.5 15 rho + D0₂₂ — — — — — — 4 75 5 17.5 2.5 D0₂₄ — —— — 72.8 4.2 5 75 5 12.5 7.5 D0₂₄ + D0₂₂ — — — — 73.6 4.8 6 75 5 7.512.5 D0₂₄ + D0₂₂ + rho — — — — ND ND 7 75 5 2.5 17.5 L1₂ + D0₂₄ + D0₂₂ND ND ND ND ND ND 8 75 7.5 12.5 5 L1₂ + D0₂₄ 73.9 9.0 12.6 4.5 73.5 6.69 75 7.5 7.5 10 D0₂₄ — — — — 74.3 7.2 10 75 7.5 2.5 15 L1₂ + D0₂₄ + D0₂₂ND ND ND ND 74.7 7.8 11 75 10 7.5 7.5 L1₂ + D0₂₄ 74.0 10.6   7.3 8.074.0 7.0 12 75 10 2.5 12.5 L1₂ + D0₂₂ ND ND ND ND — — 13 75 1.25 11.312.5 rho + D0₂₂ — — — — — — 14 75 1.25 7.5 16.25 rho + D0₂₂ — — — — — —15 75 1.25 2.5 21.25 D0₂₂ — — — — — — 16 75 2.5 15 7.5 D0₂₄ + rho — — —— 73.6 2.5 17 75 2.5 5 17.5 D0₂₂ — — — — — — 18 75 7.5 15 2.5 L1₂ + D0₂₄73.5 8.5 14.4 3.6 73.1 6.4 19 75 7.5 5 12.5 D0₂₄ — — — — 73.6 7.2 20 7510 5 10 L1₂ + D0₂₄ ND ND ND ND ND ND Test D0₂₄(Ni ₃ Ti) D0₂₂(Ni ₃ V)Rho(Ni₃Ti _(0.7) V _(0.3) ) Material (at %) (at %) (at %) No. Ti V Ni AlTi V Ni Al Ti V 1 19.5 5.3 — — — — — — — — 2 — — 73.1 1.0 7.8 18.1 73.82.4 13.8 10.1 3 — — 70.6 2.8 8.7 18.0 72.3 3.0 13.5 11.2 4 19.5 3.4 — —— — — — — — 5 13.9 7.7 72.6 2.2 6.0 19.1 — — — — 6 ND ND ND ND ND ND74.8 3.7 10.8 10.8 7 ND ND ND ND ND ND — — — — 8 13.8 6.1 — — — — — — —— 9 6.8 11.8 — — — — — — — — 10 3.5 14.0 ND ND ND ND — — — — 11 8.0 11.0— — — — — — — — 12 — — ND ND ND ND — — — — 13 — — 73   0.7 8.5 17.8 73.21.2 14.1 11.6 14 — — 73.6 1.1 6.2 19.1 73.7 1.6 13.5 11.2 15 — — 74.10.7 2.2 23.1 — — — — 16 17.2 6.8 — — — — 73.1 2.7 15.6  8.6 17 — — 73.91.8 4.3 19.9 — — — — 18 16.8 3.8 — — — — — — — — 19 5.7 13.4 — — — — — —— — 20 ND ND — — — — — — — — Note that “rho” represents rhombohedral.

From Table 1 and FIG. 1, it may be understood that phases other than theL1₂, the D0₂₂, the D0₂₄, the D0_(a) and the rhombohedral phases were notpresent in Test Materials Nos. 1 to 20. The amount of Ni of each phasewas maintained at around 75%. Further, each phase was in an equilibriumstate as a single-phase or a multi-phase. Five regions where two phaseswere present together, and two regions where three phases were presenttogether were observed. The L1₂-D0₂₂-D0₂₄ phase coexistingmicrostructure, which is present in a region of low Ti content, is ofparticular interest as a microstructure in which the constituent phasespositioned at the three vertices of the phase diagram are directlyequilibrated.

Next, the Ni₃Al—Ni3Ti—Ni₃V pseudo-ternary phase diagram at 1273 K wasdetermined in accordance with the phase diagram shown in FIG. 1.

Test Materials Nos. 1 to 20 were vacuum-sealed in quartz tubes, and eachwas subjected to a heat treatment at 1273 K for 7 days, and then wassubjected to a water-quenching. Next, in order to form the phase diagramat 1273 K, an observation of microstructure and an analysis of eachconstituent phase were performed for each of Test Materials Nos. 1 to20. The observation of microstructure was carried out using OM (OpticalMicroscope), SEM, and TEM. The analysis of the various constituentphases was carried out using SEM-EPMA (Scanning ElectronMicroscope-Electron Probe MicroAnalyzer). The results of thisobservation and analysis are shown in Table 1. The Ni₃Al—Ni₃Ti—Ni₃Vpseudo-ternary phase diagram at 1273 K obtained from this observationand an analysis is shown in FIG. 2.

The composition range surrounded by points A, B, C, D, and E shown inFIG. 2 is the region in which the multi-phase microstructure or the dualmulti-phase microstructure is obtained with certainty.

The present invention is realized by reducing the amount of V andsubstituting a portion of V with Nb within the above composition range.As a result, by providing a composition within the range surrounded bylines that connect point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at%, Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %,Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %,Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %,Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %,Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shownin FIG. 2, it is possible to obtain the targeted Ni-based compoundsuperalloy which has a multi-phase microstructure or a dual multi-phasemicrostructure with certainty.

Test materials having the compositions shown in Table 2 below wereprepared, and then the properties thereof were evaluated in order toinvestigate the composition and the microstructure of the Ni-basedcompound superalloy having the composition system according to thepresent invention, based on the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phasediagram shown in FIG. 2.

TABLE 2 at % Ni Co Cr Al Ti V Nb #21 Addition of Nb 75 12.5 2.5 7 3 #22(V is substituted) 75 12.5 2.5 5 5 #23 75 12.5 2.5 10 #24 Addition of Cr73.5 3 12.5 2.5 8.5 #25 (V is substituted) 72.5 5 12.5 2.5 7.5 #26Combined 70 5 12.5 2.5 7 3 addition of Nb, Co #27 Combined 68.5 5 3 12.52.5 8.5 addition of Cr, Co #28 Combined 68.5 5 3 12.5 2.5 5.5 3 additionof Nb, Cr, Co

Each sample having the compositions shown in Table 2 was melted andsubjected to a heat treatment at 1573 K (1300° C.) for 10 hours in avacuum furnace. This treatment corresponds to a homogenizing treatment.Next, argon gas was introduced into the furnace by means of a gas fancooling, and stirring and cooling was performed. Next, gas fan coolingwas carried out at 1373 K (1100° C.) for 10 hours (first heattreatment), and then gas fan cooling was carried out at 1273 K (1000°C.) for 10 hours (second heat treatment). Each test material was thusobtained and supplied for the following compression tests.

[Compression Test]

Test Materials Nos. 21, 22 and 28 shown in Table 2 were employed. Thecompression test was performed using square test pieces havingdimensions of 2×2×5 mm³ under conditions where the temperature is in arange of the room temperature to 1273 K, the atmosphere is vacuum, andthe strain rate is 3.3×10⁻⁴ s⁻¹. These results are shown in FIG. 3. FIG.3 shows the 0.2% yield stresses (MPa) measured at the varioustemperatures of 298 K, 673 K, 773 K, 873 K 973 K, 1073 K, 1173 K, and1273 K.

From the results of the compression tests shown in FIG. 3, it is clearthat it is possible to obtain a value of 300 MPa for the 0.2% yieldstress even at 1273 K (1000° C.), and that it is possible to obtain ayield stress value that exceeds 600 MPa in the temperature range of 300K to 1073 K. Accordingly, as for the test material according to thepresent invention, superior high-temperature strength could be attained.

[Oxidation Test]

FIG. 4 shows the results of measurements of the amount of weightincrease, including peeling, after Test Materials Nos. 21 to 28(dimensions: 10×10×10 mm) were subjected to exposure for a specific timeperiod at 1000° C. in air.

Also, in FIG. 4, the results of Test Material No. 10 (Al: 7.5%) in Table1; Test Material CMSX-4 (trade name, manufactured by Cannon-MuskegonCorp. (United States)) (Ti: 1.0 wt %, Co: 9.0, Cr: 6.5, Mo: 0.6, Al:5.6, Ta: 6.5, Hf: 0.10, rare earth (Re) 3.0, with the remainder beingNi); a test material containing Al: 14% (Al: 14%, Ti: 2.5%, V: 8.5%, Ni:75%); and a test material containing Co: 5% (Co: 5%, Al: 7.5%, Ti: 2.5%,V: 15%, Ni: 75%) are shown for comparison.

In FIG. 4, there are six different time periods for exposure noted onthe plot in order from the left: 24 hours, 50 hours, 100 hours, 200hours, 400 hours, and 500 hours.

From the results shown in FIG. 4, it is clear that an increase in weightwas suppressed for all of Test Materials Nos. 21 to 28 as compared tothe test material containing Al: 14% and the test material containingCo: 5%. Note that Test Material CMSX-4 is a well-known Ni-basedsuperalloy. However, the oxidation resistance properties of TestMaterials Nos. 22, 23, and 28 were clearly superior to this superalloy.Moreover, the oxidation resistance of Test Material No. 21 was superiorto that of the Test Material CMSX-4 in the case of time periods being400 hours or less. Further, the oxidation resistances of Test MaterialsNos. 24 and 25 were superior to that of the Test Material CMSX-4 in thecase of time periods up to 200 hours.

Further, it was clear that all of the test materials had superioroxidation resistance as compared to a test material of theNi₃Al—Ni₃Ti—Ni₃V system alloy (test material containing Al: 7.5% in FIG.4) researched by the present inventors.

[Metallographic Structure]

FIG. 5 shows a photo of a metallographic structure of Test Material No.21 (see FIG. 5(A)), a partially enlarged view (5000-fold magnification)of the photo of the metallographic structure of the same test material(see FIG. 5(B)), a photo of a metallographic structure of Test MaterialNo. 22 (see FIG. 5(A)), and a photo of a metallographic structure ofTest Material No. 23 (see FIG. 5(A)). The magnification of the photos ofthe various test materials shown in FIG. 5(A) is 100-fold, and a 100 μmwhite line is recorded in each photo for showing the magnificationscale.

In the photo of Test Material No. 21, the contrast was poor so that itwas difficult to discriminate; however, it was possible to confirm thepresence of the Ni₃Al (L1₂) phase in almost the entirely of the testmaterial. From the partially enlarged view (5000 times) of the photo ofthe metallographic structure of this test material, it was clear that adual multi-phase microstructure including a primary L1₂ phase and an(L1₂+D0₂₂) eutectoid microstructure was formed.

In the photos of Test Materials Nos. 22 and 23, the Ni₃Al (L1₂) phase isclearly confirmed; however, it is clear that the amount of theNi₃Al(L1₂) phase is reduced. When the amount of the Ni₃Al (L1₂) crystalgrains decreases as in the photo, formation of the multi-phasemicrostructure tends to become difficult. (Test Material No. 21 includesV: 7 at %, Nb 3 at % as shown in Table 2; Test Material No. 22 includesV, Nb: 5 at %; and Test Material No. 23 includes V: 0 at %, Nb: 10 at%.)

Among these metallographic structures, those that include a multi-phasemicrostructure or include a dual multi-phase microstructure do notreadily undergo large changes in microstructure even at hightemperatures. Due to this stability, a large high-temperature strengthis attained. Further, it is important to form a microstructure in whichthese multi-phase microstructures are formed as finely and as coherentlyas possible for the purpose of enabling a microstructure which hassuperior mechanical properties at even higher temperatures.

FIGS. 6 and 7 show photos of a metallographic structure of Test MaterialNo. 28 (1000-fold magnification). FIG. 8 shows a partially enlarged view(2500-fold magnification) of the photo of the metallographic structureof the same test material.

The fine granular portion in the photo of the metallographic structureshown in FIG. 6 is a L1₂-D0₂₄-D0_(a) microstructure and occupies themajority of the microstructure in the photo. When this fine granularportion is enlarged at 2500-fold magnification, it could be confirmedthat this portion becomes a microstructure in which numerous irregularNi₃Al (L1₂) crystal grains are spread out as shown in FIG. 8. Note thatit is clear that in the microstructure in which the numerous Ni₃Al (L1₂)crystal grains are spread out, L1₂-D0₂₄-D0_(a) phases exist at theboundary regions between the Ni₃Al (L1₂) crystal grains in the same wayas the test material shown in FIG. 5.

From the above photos of microstructures, it is clear that testmaterials to which the combined addition of Cr and Co as well as thecombined addition of V and Nb is employed, such as Test Material No. 28,also have a multi-phase microstructure.

Note that while a Ni₃Ti phase is observed in the lower left side of thephotos of the metallographic structures in FIGS. 6 and 7, it isdesirable that this type of coarse plate-like Ni₃Ti phase is notpresent.

[Measurement of Specific Gravity]

The specific gravity of Test Material No. 21 was 7.90. The specificgravity of Test Material No. 22 was 7.95. The specific gravity of TestMaterial No. 23 was 8.07. The specific gravity of Test Material No. 24was 7.90. The specific gravity of Test Material No. 25 was 7.87. Thespecific gravity of Test Material No. 26 was 7.88. The specific gravityof Test Material No. 27 was 7.8. The specific gravity of Test MaterialNo. 28 was 7.86. From these, it is clear that it is possible to achievea reduction in weight as compared to the typical Ni-based superalloyssuch as MarM247 (registered trademark): 8.54 g/cm³ and CMSX-4(registered trademark): 8.70 g/cm³.

Next, based on the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shownin FIG. 2, test materials having the compositions shown in Table 3 belowwere produced and the properties of those test materials were evaluatedin order to investigate the effects of the addition of Al, the effectsof the addition of Nb, the effects of the addition of Cr, and theeffects of the addition of Co, in a Ni-based compound superalloy havingthe composition system according to the present invention.

TABLE 3 at % Ni Co Cr Al Ti V Nb B Ta Comparative 74.95 7.5 2.5 15 0.05material Al 12% 75 12 2.5 10.5 Al 13% 75 13 2.5 9.5 Al 14% 75 14 2.5 8.5Cr 5% 70 5 7.5 2.5 15 Co 5% 70 5 7.5 2.5 15 Nb 3% 72 7.5 2.5 15 3 Nb 1%74 7.5 2.5 15 1

Test materials having the compositions shown in Table 3 were produced inthe same manner as the test materials shown in Table 2, and theoxidation resistance test was performed for each test material at atesting temperature of 1000° C. These results are shown in FIG. 9.

From the results shown in FIG. 9, it is clear that, with regard to theNi-based compound superalloy having the composition system according tothe present invention, a large improvement in the property of oxidationresistance cannot be achieved by just providing a composition system inwhich Co or Cr is simply added. Moreover, it is clear that the sameholds true for Al. Thus, selection of specific ranges as explained aboveis extremely important in the present invention.

Various samples having the compositions shown in Table 4 were melted andsubjected to a heat treatment at 1563 K (1290° C.) for 10 hours in avacuum furnace. This treatment corresponds to a homogenizing treatment.Next, argon gas was introduced into the furnace by means of a gas fancooling, and stirring and cooling was carried out. Next, a heattreatment was carried out at 1373K (1100° C.) for 10 hours, and then gasfan cooling was carried out (first heat treatment). Thereafter, a heattreatment was carried out at 1273 K (1000° C.) for 10 hours, and thengas fan cooling was carried out (second heat treatment). Each testmaterial was thus obtained and supplied for the following tests.

TABLE 4 Test Material Sample Composition (at %) No. Ni Co Cr Al Ti V NbZr 41 68.5 5 3 12.5 1.5 6.25 3.25 42 68.5 5 3 12.5 0.5 7 3.5 43 68.5 5 310 1.5 8 4 44 68.5 5 3 7.5 1.5 9.5 5 45 68.5 5 3 10 1.5 6 6 47 67.5 5 510 1.5 6 5 48 63.5 10 3 10 1.5 8 4 51 68.5 5 3 12.5 2 5.75 3.25 52 68.55 3 12.5 2 5.25 3.75 53 68.5 5 3 12.5 1.5 6 3.5 54 68.5 5 3 12.5 1.5 5.54 55 69 5 3 12.5 1.5 5.75 3.25 56 69 5 3 12.5 1.5 5.25 3.75 57 68.5 5 312 1.5 6.25 3.75 58 68.5 5 3 11.5 1.5 6.25 4.25 63 68.5 5 3 12.5 1.55.75 3.75 64 69 5 3 12 1.5 5.75 3.75 65 69.5 5 3 11.5 1.5 5.75 3.75 6769.5 5 3 11.5 1.5 5.75 3.75 1.5

For the test materials shown in Table 4, the specific gravity of TestMaterial No. 41 was 7.94, and the specific gravity of Test Material No.65 was 8.01. In contrast, the specific gravity of Test Material No. 10in Table 1 was 8.00. From these, it is clear that it is possible toachieve a reduction in weight for Test Materials Nos. 41 and 65 ascompared to the above-described typical Ni-based superalloys such asMarM247 (registered trademark): specific gravity is 8.54 and CMSX-4(registered trademark): specific gravity is 8.70.

FIG. 10 shows the results of oxidation tests for Test Materials Nos. 41to 48 shown in Table 4, which were obtained by measuring the amount ofweight increase including peeling after each test material (dimensions:10×10×10 mm) was subjected to exposure at 1000° C. for a specific timeperiod in air. In FIG. 10, the results for Test Material No. 10 (Al:7.5%) shown in the previous Table 1 are also shown for comparison.

From the results of the oxidation tests shown in FIG. 10, all TestMaterials Nos. 41 to 48 according to the present invention demonstratedexcellent oxidation resistance as compared to Test Material No. 10. Morespecifically, Test Materials Nos. 28, 41, 46, 42, and 47 showed, in thisorder, superior oxidation resistance.

The same oxidation tests were conducted for Test Materials Nos. 51 to 58and Test Materials Nos. 63 to 67 shown in Table 4, and the results ofTest Materials Nos. 51 to 58 are shown in FIG. 11, and the results ofTest Materials Nos. 63 to 67 are shown in FIG. 12. In FIGS. 11 and 12,the results of Test Materials Nos. 10, 28, and 41 are also included.

From the results of the oxidation tests shown in FIGS. 11 and 12, allTest Materials Nos. 51 to 58 and Nos. 63 to 67 according to the presentinvention demonstrated better oxidation resistance than that of TestMaterial No. 10. Note that Test Material No. 67 is a test material thatincludes Zr in the amount of 1.5 at %, in addition to prescribed amountsof Co, Cr, Al, Ti, V, and Nb. Test Material No. 67 demonstratesoxidation resistance properties which are superior to those of TestMaterial No. 10. Accordingly, it became clear that a Ni-based compoundsuperalloy having superior oxidation resistance can be obtained in thecase of a composition system in which Zr is added to the compositionaccording to the present invention.

Next, the results of tensile tests carried out on Test Materials Nos.28, 41, and 65 shown in Tables 2 and 4 are shown in FIG. 13. The testmaterials used in these tensile tests are test materials in which boron(B) was added in the amount of 100 ppm for substituting Ni. From thesetests, it may be understood that, while the tensile strength of TestMaterials Nos. 28, 41 and 65 was slightly less than that of the TestMaterial No. 10 in the temperature range from the room temperature to700° C., the rates of reduction in tensile strength for Test MaterialsNos. 28, 41 and 65 were less than that of Test Material No. 10 in thetemperature range from more than 700° C. to 1000° C. Further, TestMaterials Nos. 28, 41 and 65 demonstrated a higher strength than that ofTest Material No. 10 in the temperature range from 800 to 1000° C.Accordingly, it is clear that the Ni-based compound superalloy accordingto the present invention is suitable as a structural material requiredto have high-temperature heat resistance, such as for an engine or thelike where high-temperature strength is particularly demanded.

From among the test materials shown in Table 4, photos of metallographicstructures of Test Materials Nos. 41, 47, 48, 52, 57 and 65 are shown inFIGS. 14 to 22.

FIG. 14 shows a photo of a metallographic structure in which the surfaceof Test Material No. 41 is enlarged at 1000-fold magnification. FIG. 15shows a photo of a metallographic structure in which the surface of thesame test material is enlarged at 5000-fold magnification. As in thecase of the photos of the metallographic structures of the testmaterials shown in FIGS. 6 and 8, the fine granular portions in thephotos of the metallographic structures are the L1₂-D0₂4-D0_(a)microstructures and occupy the majority (entirety) of themicrostructures in the photos. When this fine granular portion isenlarged at 5000-fold magnification, it could be confirmed that thatthis portion becomes a microstructure in which numerous irregularNi₃Al(L1₂) crystal grains are spread out as shown in FIG. 15. Note thatit is clear that in the microstructure in which the numerous Ni₃Al (L1₂)crystal grains are spread out, L1₂-D0₂₄-D0_(a) phases exist at theboundary regions between the Ni₃Al(L1₂) crystal grains in the same wayas the previous test material. Note that the magnification scaleindicated by the 11 white points shown in FIG. 14 is 30 μm, and themagnification scale indicated by the 11 white points shown in FIG. 15 is6 μm.

FIG. 16 shows a photo of a metallographic structure in which the surfaceof Test Material No. 47 is enlarged at 5000-fold magnification. FIG. 17shows a photo of a metallographic structure in which the surface of TestMaterial No. 48 is enlarged at 5000-fold magnification. FIG. 18 shows aphoto of a metallographic structure in which the surface of the TestMaterial No. 52 is enlarged at 2500-fold magnification. FIG. 19 shows aphoto of a metallographic structure in which the surface of TestMaterial No. 57 is enlarged at 2500-fold magnification. FIG. 20 shows aphoto of a metallographic structure in which the surface of TestMaterial No. 65 is enlarged at 50-fold magnification. FIG. 21 shows aphoto of a metallographic structure in which the surface of TestMaterial No. 65 is enlarged at 100-fold magnification. FIG. 22 shows aphoto of a metallographic structure in which the surface of TestMaterial No. 65 is enlarged at 5000-fold magnification. Note that themagnification scales indicated by the white lines shown in FIGS. 16 and17 are 5 μm; the magnification scales indicated by the white lines shownin FIGS. 18 and 19 are 10 μm; the magnification scale indicated by thewhite line shown in FIG. 20 is 500 μm; the magnification scale indicatedby the white line shown in FIG. 21 is 10 μm; and the magnification scaleindicated by the white line shown in FIG. 22 is 5 μm.

From these photos of the metallographic structures, it is clear that thefine granular portion in the photo of the metallographic structure isthe L1₂-D0₂₄-D0_(a) microstructure and occupies the majority (entirety)of the microstructure in the photo for each of Test Materials Nos. 47,48, 52, 57 and 65.

FIG. 23 shows the results of tensile testing at room temperature fortest materials that were prepared by adding various amounts of boron toTest Material No. 65 for substituting Ni. For the test material shown inFIG. 23, there was absolutely no plastic elongation, and the tensilestrength was low in the case when no (0 ppm) boron was added. In thecase in which the added amount of boron was increased to 25 ppm, theelongation increased, plastic elongation was demonstrated, and thetensile strength increased. However, in the case in which boron wasadded in excess of the upper limit of 1000 ppm, any plastic elongationwas not demonstrated again, and the fracture strength was low. Fromthese results, it is desirable that the amount of boron added to thesuperalloy according to the present invention is 0 ppm or more to 1000ppm or less, or less than 1000 ppm from the perspective of elongation.

FIG. 24 shows the photo of a metallographic structure (3000-foldmagnification, white line magnification scale: 5 μm) for a test materialwhich was obtained by adding 25 ppm of boron to Test Material No. 65 andwas subjected to a homogenizing treatment at 1300° C. for 3 hours. FIG.25 shows the photo of a metallographic structure (3000-foldmagnification, white line magnification scale: 5 μm) for a test materialwhich was obtained by adding 25 ppm of boron to Test Material No. 65 andwas subjected to a homogenizing treatment at 1330° C. for 3 hours. Thesetest materials were subjected to the homogenization treatment at 1300°C. or 1330° C. for 3 hours, and then were cooled. Thereafter, both ofthem were subjected to a same heat treatment which includes a process ofheating including heating at 1100° C. for 10 hours and then cooling, anda process of heating including heating at 1000° C. for 10 hours and thencooling.

As is clear from comparing FIGS. 24 and 25, when the temperature of thehomogenizing heat treatment performed on the test materials relating toTest Material No. 65 are increased, it is possible to make themicrostructure finer. Further, it can be assumed that the effect ofimproving the tensile strength is attained by making the microstructurefiner.

INDUSTRIAL APPLICABILITY

The Ni-based compound superalloy according to the present invention canbe employed as a structural material where high-temperature heatresistance is required, such as for an engine. The Ni-based superalloyaccording to the present invention has a slightly lower specific gravitythan those of conventional Ni-based superalloys, and has superior inoxidation resistance and excellent tensile strength at hightemperatures. As a result, an improvement in engine efficiency can beattained in the engine in which the Ni-based compound superalloyaccording to the present invention is employed.

1. A Ni-based compound superalloy having excellent oxidation resistance,comprising: Al: more than 5 at % to 13 at % or less; V: 3 at % or moreto 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, withthe remainder being Ni and unavoidable impurities, and having amulti-phase microstructure comprising a primary L1₂ phase and an (L1₂phase+D0₂₂ phase and/or D0₂₄ and/or D0_(a) phase) eutectoidmicrostructure.
 2. The Ni-based compound superalloy according to claim1, wherein the Ni-based compound superalloy further comprises Nb: 3 at %or more to 9.5 at % or less, and the amount of V is not less than theamount of Nb.
 3. A Ni-based compound superalloy having excellentoxidation resistance, having a multi-phase microstructure comprising aprimary L1₂ phase and an (L1₂ phase+D0₂₂ phase and/or D0₂₄ and/or D0_(a)phase) eutectoid microstructure, which has a composition within thelimits which link point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %,Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %,Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %,Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %,Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %,Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shownin FIG.
 2. 4. The Ni-based compound superalloy having excellentoxidation resistance according to claim 2, wherein the Ni-based compoundsuperalloy further comprises at least one or more of Co: 15 at % or lessand Cr: 5 at % or less.
 5. The Ni-based compound superalloy havingexcellent oxidation resistance according to claim 4, wherein theNi-based compound superalloy further comprises B: 1000 ppm (weight) orless.
 6. The Ni-based compound superalloy according to claim 1, whereinthe Ni-based compound superalloy has a dual multi-phase microstructureincluding the primary L1₂ phase and the (L1₂ phase+D0₂₂ phase and/orD0₂₄ and/or D0_(a) phase) eutectoid microstructure.
 7. A heat-resistantstructural material having excellent oxidation resistance, comprisingthe Ni-based compound superalloy according to claim
 1. 8. A method formanufacturing a Ni-based compound superalloy having excellent oxidationresistance, the method comprising: subjecting an alloy materialcontaining Al: more than 5 at % to 13 at % or less; V: 3 at % or more to9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with theremainder being Ni and unavoidable impurities, to a first heat treatmentat a temperature at which a primary L1₂ phase and an Al phase coexist;and thereafter cooling the alloy material to a temperature at which theprimary L1₂ phase and a D0₂₂ phase and/or a D0₂₄ phase and/or a D0_(a)phase coexist, or further subjecting the alloy material to a second heattreatment at this temperature, thereby converting the Al phase to an(L1₂ phase+D0₂₂ phase and/or D0₂₄ phase and/or D0_(a) phase) eutectoidmicrostructure to form a multi-phase microstructure.
 9. The method formanufacturing a Ni-based compound superalloy according to claim 8,wherein the alloy material further comprises Nb: 3 at % or more to 9.5at % or less, and the amount of V is not less than the amount of Nb. 10.A method for manufacturing a Ni-based compound superalloy havingexcellent oxidation resistance, the method comprising: subjecting analloy material having a composition within the limits which link point A(Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B(Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C(Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D(Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and pointE (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in theNi₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2, to afirst heat treatment at a temperature at which a primary L1₂ phase andan Al phase coexist; and thereafter cooling the alloy material to atemperature at which the primary L1₂ phase and a D0₂₂ phase and/or aD0₂₄ phase and/or a D0_(a) phase coexist, or further subjecting thealloy material to a second heat treatment at this temperature, therebyconverting the Al phase to an (L1₂ phase+D0₂₂ phase and/or D0₂₄ phaseand/or D0_(a) phase) eutectoid microstructure to form a multi-phasemicrostructure.
 11. The method for manufacturing a Ni-based compoundsuperalloy having excellent oxidation resistance according to claim 8,wherein the alloy material further comprises at least one or more of Co:15 at % or less, and Cr: 5 at % or less.
 12. The method formanufacturing a Ni-based compound superalloy having excellent oxidationresistance according to claim 8, wherein the alloy material furthercomprises B: 1000 ppm or less.
 13. The method for manufacturing aNi-based compound superalloy having excellent oxidation resistanceaccording to claim 8, wherein the first heat treatment is carried out ata temperature at which the alloy material is in a first state shown inFIG.
 1. 14. The method for manufacturing a Ni-based compound superalloyhaving excellent oxidation resistance according to claim 8, wherein thesecond heat treatment is carried out at 1173K to 1273K.